1. Field
This invention relates to a device for transporting control rod assemblies between fuel assemblies in a facility associated with a nuclear power generation plant and, more specifically, to a device for transferring control rod assemblies among fuel assemblies that uses an overhead crane as its primary lifting mechanism.
2. Related Art
Fuel for a nuclear reactor used to create steam and, ultimately, electricity, generally is in the form of fuel rods containing a fissile material. When fuel rods are being stored, the fuel rods are typically supported in nuclear fuel assemblies arranged as spaced parallel arrays. Fuel assemblies are stored in racks in a protective medium, such as water containing boric acid. In addition to the fuel rods, poison rods and/or water displacer rods are dispersed throughout the fuel assemblies to control the fission process. Poison rods generally include a plurality of elongated rods, each containing a neutron absorbent material, which fit in longitudinal openings, or thimbles, defined in the fuel assemblies. The top end of each poison rod is attached to a web or spider, thereby forming a poison rod assembly. A T-shaped bar or threaded hub is affixed to the top of the web or spider creating an easily accessible attachment point for lifting a poison rod assembly so it may be transferred from one fuel assembly to another.
Because the poison rods, which are approximately 12 to 14 feet (3.66 to 4.27 meters) long, are only connected by the web or spider located at the top of the poison rod assembly, the lower end of the poison rods move freely, and, as such, can be difficult to reinsert within a second fuel assembly without the aid of a means for alignment. Prior art transfer devices provided comb assemblies as an alignment means. A comb assembly consists of at least two plates having slots corresponding to the rows of poison rods in the poison rod assembly. The plates are oriented at 90° to each other. As such, when the plates are positioned on top of each other, a grid is formed with an opening for a poison rod at each intersection of the slots. To provide extra stability, comb assemblies typically have two spaced grids. Each comb grid is typically formed of four plates, two plates with channels extending in one direction and two plates with channels extending in perpendicular direction. The plates are divided so that the comb assemblies may be moved aside while the web portion of the poison rod assembly is being lifted from the fuel assembly. Once the web portion of the poison rod assembly is above the comb assemblies, the comb assemblies are brought into place providing support and guidance for the poison rods.
As shown in Hornack et al., U.S. Pat. No. 5,325,408, some prior art transfer devices use a winch located at the top of the transfer device to lift the poison rod assembly. Movement of the transfer device itself between fuel cells, is accomplished by a gantry crane in the fuel cell storage facility. Thus, this type of prior art lifting device consists of at least two lifting means, one to lift the transfer device itself, and one to lift the poison rod assembly within the transfer device. Burton et al., U.S. Pat. No. 6,327,322, recognize the advantage of the transfer device functioning with a single lifting mechanism. However, movement of the transfer device was still awkward because of the extended length of the transfer mechanism which was suspended from the gantry crane.
According, a further improvement in the transfer mechanism is desired that will improve its reliability and ease of operation.